Characterizing the Coronal Properties of Main Sequence Stars with Chandra/LETGS Spectra

We analyze the X-ray spectra of 19 main sequence stars observed by Chandra/LETGS.  Emission measure (EM) distributions and coronal abundances are measured from emission lines in the spectra.  We find that the EM distributions correlate with surface X-ray flux (F_X) in a predictable way, regardless of spectral type.  Thus, we provide EM distributions as a function of F_X that can be used to estimate the EM distribution of any main sequence star with a measured broadband X-ray luminosity.  Unlike the EM distributions, coronal abundances are very spectral-type dependent.  Surprisingly tight correlations with surface temperature are seen for both relative and absolute abundances.  The fractionation of elements during coronal heating is a crucial diagnostic for the coronal heating process, so exploring the spectral type dependence of this fractionation is potentially a fruitful avenue for testing different heating mechanisms.  Finally, we note that the exoplanet host star Tau Boo has anomalous coronal abundance characteristics, suggesting that the presence of a close-in massive exoplanet may be affecting the star's coronal abundances.